Jonathan Marchessault scored, Colin Miller and Alex Tuch each had two assists, and Marc-Andre Fleury made 27 saves for the Golden Knights (7-8-1).
"The puck finally went in the net for us," Vegas coach Gerard Gallant said. "So that was really a key for us, to get five goals tonight. Real happy about that, and I thought, for the most part, we had a real good game."
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Thomas Chabot had a goal and an assist, Matt Duchene and Ryan Dzingel scored, and Craig Anderson made 37 saves for Ottawa (6-7-3), which trailed 3-0.
"We just had a bad start, myself included," Dzingel said. "You can't give a team like that a couple-goal lead. It's tough when you battle back, too, after that. So [we've] just got to have a better start, myself included. You've got to be ready to go."
The Golden Knights took a 4-3 lead when William Carrier backhanded a shot past the blocker of Anderson at 9:16 of the third period.
Bellemare scored into an open net when Anderson was down to make it 5-3 at 12:00. The Senators challenged for goaltender interference, but it was ruled that Anderson initiated contact with Carrier.
"[The Carrier-Bellemare-Reaves line] played well," Gallant said. "They were setting the pace again for us tonight, they played real good. So they deserve everything they get."